Water Activity (aw) in Foods

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—it’s about available moisture. Technically, it’s the ratio of a food’s vapor pressure to that of pure water under the same conditions. For example, an aw of 0.80 means the food’s vapor pressure is 80% that of distilled water. As temperature rises, so does water activity.

Why does this matter? Because microbial growth depends on it. Most foods have an aw above 0.95, which is high enough to support bacteria, yeast, and mold. But when aw is reduced—through drying, formulation, or packaging—microbial growth can be inhibited. That’s why controlling aw is a key strategy in shelf-life extension and food safety.

Bonus compliance insight: Foods with a finished aw of 0.85 or lower are exempt from the acidified and low-acid canned food regulations under 21 CFR Parts 108, 113, and 114.

Would you like a visual widget or interactive chart to help clients interpret aw values by product type? I can help design one.

Not all water in food is available for microbes to use. While bacteria depend on free, mobile water to transport nutrients through their cell walls, much of the water in food is bound to molecular sites—such as hydroxyl groups in carbohydrates or amino groups in proteins. This bound water doesn’t act as a solvent and cannot support microbial growth.

The way food holds onto water is called its sorption behavior. Scientists study this using sorption isotherms—curves that describe how moisture content relates to partial pressure or water activity at a constant temperature. These curves reveal how water behaves under equilibrium conditions, where the vapor pressure of the food matches that of the surrounding air.

• Closed chamber method – Food samples equilibrate in a sealed chamber; water activity is measured with hygrometers or dewpoint cells.
   
• Humidity exposure method – Samples are exposed to controlled humidity environments (often via saturated salt solutions); moisture content is measured after equilibrium.
   

• Drying and concentration processes – Understanding sorption shows how tightly water is bound and how easily it can be removed.
   
• Shelf life and stability – Controlling final water activity helps prevent spoilage and ensures products remain safe during storage.
   
• Packaging decisions – Knowing equilibrium behavior informs barrier requirements for moisture-sensitive foods.
   

By understanding sorption behavior, food producers can better predict microbial safety, product quality, and storage stability.

 Water activity (aw) is a critical measure of how tightly water is bound within a food product—and how available it is for microbial growth. Technically, it’s the ratio of the vapor pressure of water in the sample to that of pure water at the same temperature. Our lab uses an Aqualab brand meter to measure aw, and we report results to the first two significant digits (e.g., 0.72). 🧪 How We Measure We use sealed sample chambers with sensors positioned above the food to measure water activity under controlled conditions. Samples are typically tested in duplicate for accuracy. Calibration is verified using traceable salt slush standards (e.g., sodium chloride, potassium nitrate, potassium sulfate), which span a water activity range from 0.25 to 1.00 at 25°C. Temperature control is essential. Even a 0.1°C difference between the sample and surrounding air can shift the aw reading by ~0.005. A full 1°C variation could cause a 0.05 shift—enough to misclassify a product’s safety threshold, especially near the critical limit for C. botulinum (aw ≈ 0.93). That’s why we maintain strict thermal equilibrium during testing. ⚙️ Sample Requirements & Turnaround - Sample size: 25g - Data format: aw units - Method: AOAC 978.18 - Turnaround: 3 business days (rush testing available) - Note: Samples high in propylene glycol may not be suitable for this method. 🧠 Why It Matters Water activity influences shelf life, microbial safety, and regulatory classification. It’s especially important when formulating products for preservation—ingredients must be precisely balanced to achieve the target aw at the time of packing.